vfio: Split the container logic into vfio_container_attach_group()

This splits up the ioctl of vfio_group_ioctl_set_container() so it
determines the type of file then invokes a type specific attachment
function. Future patches will add iommufd to this function as an
alternative type.

A following patch will move the vfio_container functions to their own .c
file.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/3-v3-297af71838d2+b9-vfio_container_split_jgg@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>